326 BELL SYSTEM TECHNICAL JOURNAL 



I have given above, the permitted values of the angular momentum 

 of this orbit of the valence-electron. Now I point out that to each 

 of these permitted values of p corresponds a permitted value of the 

 magnetic moment /i, which I obtain by multiplying the former with 



ejlmc: 



n = (1, 2, 3, 4, • • •){eh/47rmc). 



However, only one (at most) of these values can be appropriate to 

 the normal state of the sodium atom; all the rest must correspond to 

 abnormal, unusual, or "excited" states. We are going to be inter- 

 ested primarily in the normal state, so we must identify the right one. 

 In the early days of the Bohr theory, the right one was supposed to 

 be the first which I have written down. However, the theory has 

 been greatly remodelled and bettered since those days, with the aid 

 of what is known as "quantum mechanics"; and it now seems quite 

 certain that these lists of the permitted values of p and fj, for electron- 

 orbits are both incomplete. I must add to each of them the value 

 zero, so that the two lists become 



p = (0, 1, 2,3,4, ■■■)hl2ir, 

 /JL = (0, 1, 2, 3, 4, • • •)eh/4:Trmc, 



Moreover, it is precisely this new value zero which belongs to the 

 normal state of the sodium atom. So the theory, in this stage, quite 

 definitely prescribes that the sodium atom in its normal state should 

 have no magnetic moment and no angular momentum. But now let 

 us look at the data. 



It would do no good in this connection to make measurements on 

 solid or on liquid sodium, for in those "condensed phases" the atoms 

 are crowded so closely together as to be badly distorted. We can, 

 however, experiment on free atoms of sodium, in their normal state, 

 in the way illustrated by Fig. 1. In the upper portion, A repre- 

 sents an "oven," consisting of a small box heated electrically and 

 containing some liquid sodium which is steadily being vaporized. 

 There is a hole in the wall of the box through which free sodium atoms 

 are steadily shooting in all directions, with the distribution-in-speed 

 which we know from the theory of thermal agitation; and beyond, 

 there is a sequence of diaphragms with slits in them which delimit a 

 straight and narrow beam of these fast-moving atoms . Disregarding 

 what the theory has just said, let us suppose that each of these atoms 

 is a magnet — a bar magnet, with a north pole and a south pole. As 

 they emerge from the oven, these atoms must surely be oriented at 

 random in all directions. 



